What was the reason for King Asoka’s promulgation of Buddhism?

One of the most important events in the history of Buddhism took place in this period, namely, King Asoka’s promulgation of the Buddha Dhamma. Prior to King Asoka, the spread of Buddhism had been confined to the Gangs Valley in central India. Owing to King Asoka’s efforts, Buddhism became a world-wide religion, not only spreading over the five parts of India (Eastern, Western, Southern, Northern and Central India) but also extending to many countries in Asia, North Africa and Greece. (From Essentials of Buddhism: Questions and Answers)